Degenerate four-wave mixing experiments on GaN in the quasistationary regime

In order to determine the third order of the nonlinear susceptibility χ(3) of GaN, we perform degenerate four-wave mixing measurements in a two-beam configuration at low temperature on a GaN epilayer on sapphire substrate. We measure the excitation spectrum of χ(3) close to the exciton resonances. Besides the χ(3) contribution, higher orders of the nonlinear susceptibility show already up around 10 kW cm−2, leading to a saturation of the signal. Results are complemented by a study of the influence of a band-to-band excitation (ℏωP=4.02 eV) on the nonlinear susceptibility.